A bolometer is a device for measuring the intensity of electromagnetic radiation of a specific wavelength range (approx. 3-15 μm). It comprises an absorber, which converts electromagnetic radiation to heat, and a device for measuring an increase in temperature. Depending on a thermal capacity of the material, there is a direct connection between an amount of radiation absorbed and the resulting increase in temperature. Thus, the increase in temperature may serve as a measure of an intensity of incident radiation. Of particular interest are bolometers for measuring infrared radiation, which is where most bolometers have a highest level of sensitivity.
A bolometer may be used, in technology, as an infrared sensor, an imager for a night-vision device or as a thermal imaging camera.
A bolometer serving as an infrared sensor comprises a thin layer which is arranged within the sensor in a thermally insulated manner, e.g. is suspended as a membrane. The infrared radiation is absorbed within this membrane, whose temperature increases as a result. If this membrane consists of a metallic or advantageously a semiconducting material, the electrical resistance will change depending on the increase in temperature and on the temperature coefficient of resistance of the material used. Exemplary values regarding various materials can be found in the paper R. A. Wood: “Monolithic silicon microbolometer arrays,” Semiconductor Semimetals, vol. 47, pp. 43-121, 1997. Alternatively, the membrane is an insulator (silicon oxide or silicon nitride) onto which the resistor has been deposited as a further thin layer. In other implementations, insulating layers and an absorber layer are disposed in addition to the resistive layer.
The temperature dependence of metal layer resistances is linear, semiconductors as resistance material have an exponential dependence. A high level of dependence is also to be expected from diodes as thermal detectors with their current/voltage characteristic in accordance withID=I0*(Exp{eUD/kT}−1)wherein T is the temperature, k is the Boltzmann constant, e is the elementary electric charge, ID and UD designate a current intensity and voltage within the diode, and I0 is a constant which is independent of the voltage.
Bolometers may serve as individual sensors, but may also be designed as rows or 2D arrays. Rows and arrays nowadays are typically produced using Microsystems engineering methods in surface micromechanics on a silicon substrate. Such arrays are referred to as microbolometer arrays.
An advantageous wavelength of the infrared radiation to be detected is about 8-14 μm, since this wavelength range comprises radiation of matter which has approximately room temperature (300 K). The wavelength range of 3-5 μm is also of interest because of a permeable atmospheric window.
An essential advantage of thermal bolometers over other (photonic) IR detectors (IR=infrared) is that they may be operated at room temperature, i.e. uncooled.
The aim of further development is to arrange as many bolometer cells (pixels) as possible within one array. Thus, the array will have a higher number of pixels and will provide a better resolution of an image at the same total area (chip area) of the array. For example, an arrangement of 160×120 pixels is customary, 320×240 is also available, 640×480 pixels (VGA resolution) has been announced and will be available shortly, but only at considerable additional cost. At the same time it is useful to minimize the cost of the array so as to open up new markets, e.g. the field of motor vehicles.
The usual dimensions of a single pixel within the array comprise a pixel area of 35 ×35 μm2 to 50 ×50 μm2. With 320 ×240 pixels, a chip area thus is at least 12.2 ×8.4 mm2 =94 mm2 (pixel area alone) plus an area for a readout circuit (e.g. an additional 2 mm per edge), in total approx. 137 mm2. Since a yield (the number of good chips in relation to the total number on a disk, or wafer) sharply decreases as the chip area increases, economic production of such an array is hardly possible. Therefore, an increase in the number of pixels should entail a reduction of the pixel area. IR imagers of 35 ×35 μm2 have been commercially available for some time now. As is described in the paper Mottin; “Above IC Amorphous Silicon Imager Devices;” Leti 2005 Annual Review; Jul. 6, 2005, pp. 1-18, arrays of 25×25 μm2 are currently being developed. But even this surface area, which has already been scaled, leads to an unacceptably large chip area (estimated to be approx. 250 mm2) with imagers exhibiting VGA resolution. Further scaling of the pixel area is therefore absolutely essential. What is aimed at are pixels having pixel areas of approx. 15×15 μm2. Further reduction in size will then conflict with the fact that the optical systems which would be employed in such a case would have to be of very high quality, which, in turn, would only be feasible at very high cost.
Detection of infrared radiation within a microbolometer is based on the fact that the radiation heats a resistor which is thermally well insulated. Said resistor is temperature-dependent, and therefore it changes its resistance as a function of warm-up. A change in resistance is read out via an ROIC (read out integrated circuit). Typical increases in temperature occurring at the resistor are within the range of several millikelvin (mK) per degree of temperature change in a target observed. For this increase in temperature at the bolometer to become possible, the resistor must be very well insulated thermally. This is achieved by arranging the resistor on a membrane (or by configuring it as a membrane itself) which is arranged, at a distance of several μm, above a disk surface and is connected to the disk surface, or to a substrate, only at few points having low thermal conductivity.
FIG. 5 shows two bolometers in accordance with the prior art which are described in R. A. Wood: “Monolithic silicon microbolometer arrays,” Semiconductor Semimetals, vol. 47, pp. 43-121, 1997. FIG. 5a depicts a single-level pixel which comprises a sensor 51, electronics 52 located on a substrate 54, and which has a pixel size 53. FIG. 5b shows a two-level pixel, wherein the electronics 52 are arranged below the sensor 51. This bolometer also corresponds to the prior art, and by comparison with the bolometer shown in FIG. 5a it comprises a higher fill factor (ratio of IR-sensitive area to the total area).
The membrane is generated, for example, in that the resistor or sensor 51 is produced on a disk surface 55, and in that subsequently, the region is undercut, so that a cavity 56 results. By locally removing silicon (Si), for example, the thermal resistance between the resistor on the membrane 51 and the substrate 54 will increase. A readout circuit 52 is integrated next to the membrane 51, and therefore takes up additional chip area. Therefore, a structure of FIG. 5b, wherein the resistor 51 is disposed in a second plane on a membrane above the readout circuit 52, is more advantageous.
For measuring the resistance, two contact points are necessary. They may be formed by arranging feed lines on portions of the membrane which incline in the upward direction. The inclinations at the same time serve as spacers for the membrane. FIG. 6 shows a perspective view of a corresponding structure comprising a membrane 10, which consists of a support 35 and a resistive layer 18. Such an arrangement is described in FIG. 2 of U.S. patent U.S. Pat. No. 5,688,699 (Nov. 2, 1997; B. T. Cunningham, B. I. Patel: “Microbolometer”). The membrane 10 is supported by inclined support arms 20 comprising an electrically conducting layer 32 and a thermally insulating layer 22. A contact of the membrane 10 via the support arms 20 comprises an overlap 33, and the support arms 20 extend into an epitaxial layer 14, where the corresponding circuit (not shown in the figure) is located. The epitaxial layer 14 is positioned between a substrate 12 and an insulating layer 24.
If the membrane 10 is planar (has no inclinations), the signals are supplied via metallic plugs which at the same time serve as spacers. This structure is described in Tissot: “Uncooled Thermal Detectors for IR Applications;” Leti 5th Annual Review; 2003, 11 pages and FIG. 7 shows a perspective view of such a conventional structure having a membrane 10 on two contact plugs 26a and 26b, which is held at a distance 72 above a foundation 73. The membrane 10 having a size 75 comprises a thickness 74, and the foundation 73 comprises a reflector. Thermal insulation from the foundation 73 is established via the bridges 76a,b. The foundation 73 has an ROIC input pad 77 located thereon by means of which the bolometer is contacted. Contacting of the membrane 10 comprises an overlap 78 as compared to a diameter of the contact plugs 26a and 26b. This overlap 78 reduces the fill factor.
Optimum absorption of the IR radiation is achieved in that the membrane 10 comprises a layer resistance in accordance with a spreading resistance of an electromagnetic wave in air (377Ω/□), and is arranged at a height of λ/4 (approx. 2.5 μm at the advantageous wavelength λ of, e.g., 8-14 μm) above a reflector 73.
US patent U.S. Pat. No. 5,912,464 cites such a bolometer and a production method, and FIG. 8 shows a portion of it. FIG. 8a shows a cross section through a contacting of the membrane 10, the cross-sectional plane being shown by a dash-dotted line in FIG. 8b with a viewing direction 81.
The contact plug 26b contacts a terminal pad 77, and, at the same time, a contact layer 23. Further layers of the bolometers are a reflection layer 21, a sacrificial layer 22, the bolometer or resistive layer 27, and transition layers 24 and 25. The electrical contacting of the resistive layer 27 is established via the contact layer 23, and the transition layers 24 and 25 serve for improved contacting of the contact layer 23. The contact layer 23 extends in a meandering manner along the resistive layer 27 from a contact plug 26a to the contact plug 26b. The meandering implementation of the electrode layer 23 is shown by a dashed line in FIG. 8b. The meandering implementation of the electrode layer 23 serves to improve the absorption of the infrared radiation.
It is also in this bolometer in accordance with the prior art that the contact plug 26b and the membrane 10 comprise an overlap. In FIG. 8a, the overlap of the contact plug 26b is marked by x, and the overlap of the membrane 10 is marked by y. The sacrificial layer 22 is only present in the intermediate step shown here, and will be removed later on.
With corresponding processing, a sacrificial layer 22 of polyimide is applied as a spacer to a disk having an integrated circuit (e.g. in CMOS technology; not depicted in the figure). In the region of the contact plugs 26a,b, the sacrificial layer 22 is opened in the form of a contact hole. In one implementation, which is shown in FIG. 8a, a metallic contact layer 25 is deposited and patterned, and subsequently a contact metal for the contact plugs 26a,b is deposited. This metal is etched such that it will overlap an edge of the contact hole. The resistive layer 27 is deposited and patterned. At last, the sacrificial layer 22 underneath the membrane 10 is removed, so that said membrane, which is held by the contact plugs 26a,b, is suspended above the reflection layer 21, and, thus, a λ/4 absorber is formed.
FIG. 9 shows a conventional contacting as is also used in the example of FIG. 8. The contact plug 26b comprises an overlap x over a diameter z of the contact plug 26b, and the membrane 10 comprises an overlap by a value of y over the contact plug 26b. 
All embodiments described in U.S. patent U.S. Pat. No. 5,912,464, but also the structures in accordance with U.S. patent U.S. Pat. No. 5,688,699 or of document Tissot: “Uncooled Thermal Detectors for IR Applications;” Leti 5th Annual Review; 2003, 11 pages have in common that the contact metal projects beyond the diameter z of the contact plug 26b (distance x in FIG. 9). The membrane 10 itself projects even further beyond (distance y in FIG. 9). The overlaps x and y represent a compensation for adjustment tolerances, they make sure that the region of the contact plug (the contact area in FIG. 7) is not etched.
FIG. 10 shows how the bolometers in accordance with the prior art scale when the pixel size 75 is reduced. FIG. 10a shows a top view of the membrane 10 with conventional contacting by means of the contact plugs 26a and 26b, the membrane 10 being connected to the contact plugs 26a,b via the bridges 76a,b. The bridges 76a,b act as thermal insulation. As is explained in FIG. 9, the membrane 10 overlaps the contact plug 26b by the value of y, and the contact plug 26b overlaps the diameter z of the contact plug 26b by the value of x. In case of a reduction (scaling) of the pixel size 75, as is shown in FIG. 10b, the size of the contact plugs is not scaled for technological reasons, and the fill factor decreases accordingly. A reason for this is that the conventional manufacturing process is based on photosensitive polyimide as the sacrificial layer 22, and is therefore limited to a minimum hole size which must be larger than approx. 3 μm (please see further comments below).
FIG. 10a also shows that, as is also visible in FIG. 7, the contact plugs 26a,b with their contact to the membrane 10 are indeed relatively large, but that with a pixel of an edge length of approx. 50 μm, the surface percentage thereof is relatively small. However, it may already be seen from FIG. 6 that the actual membrane area 35 only makes up for a relatively small proportion of the total area of the pixel, and that in this implementation, the fill factor is below 50%.
As may be seen in FIG. 6, FIG. 8b or FIG. 10a, the contact plug 26b is connected to the membrane 10 via a thin arm 20, or 76b. In addition to providing mechanical support and electrical supply, the arm 20, or 76b, also serves to thermally insulate the membrane 10 from the contact plug 26b. Its long length and its small cross-sectional area ensure a high thermal resistance between the membrane 10 and the substrate.
As was already described, it is desirable to make the pixels as small as possible. A direct comparison of FIGS. 10a and 10b shows that no satisfactory solution may be found for this issue with pixels of conventional technology. With the scaled pixel in FIG. 10b, the contact plugs 26a,b take up a disproportionately large share in the total pixel area. This is due to the fact that the metal of the plug projects beyond its opening through the membrane 10 by x, additionally, the membrane 10 is typically larger than the overlap x by a factor of y. With a predefined total area, the proportion of an active area on the membrane 10 becomes smaller, the fill factor decreases, and a sensitivity of the pixels to the IR radiation also decreases as a consequence.